Zoom lenses having large zoom ratios are highly desirable. Zoom lenses with 2.times. to 3.times. zoom ratios are known. Many of them have relatively few lens elements (three to five) arranged into only two or three zooming lens units. U.S. Pat. No's. 5,327,290; 5,446,592 and 5,218,478 disclose such zoom lenses. However, these zoom lenses do not provide large enough zoom ratios and their structures can not be easily modified to provide larger zoom ratios.
Zoom lenses with larger zoom ratios (4.times., 5.times. or greater) are also known. These zoom lenses are large, complex, expensive and have many lens elements. For example, U.S. Pat. No. 5,296,969 discloses a zoom lens with nine lens elements, U.S. Pat. No. 5,231,541 discloses a zoom lens with eight lens elements, U.S. Pat. No.5,235,466 discloses a zoom lens with ten lens elements, U.S. Pat. No. 5,241,420 discloses a zoom lens with seventeen lens elements and U.S. Pat. No. 5,247, 393 discloses a zoom lens with eleven lens elements. These zoom lenses have a small focal length and are principally applicable to CCD and video cameras with a small image format. If these zoom lenses were scaled up to cover a 135 mm image format, they would become very large and quite cumbersome to use.